Prompt: Quirrel is both more enthusiastic and more successful in his attempts to kill Harry. The thing is, you can't destroy a Horcrux by mundane means.
All Harry remembers of the fall is a wrenching in his stomach, and Hermione screaming. They tell him, afterwards, that his shoelace caught on the stonework of the moving staircases, and yanked him off the edge, and he fell to the bottom of Gryffindor Tower, and then he bounced.
"Wicked," he says.
Ron asks if Harry can bounce after every fall.
One broken ankle and a trip to the infirmary later, they have an answer: no. No he cannot. Hermione delivers a lecture so scathing neither of them quite dares trying again with a bigger drop than the one produced by Harry jumping off of Hagrid's hut.
Oliver thinks it's great. "You don't have to be scared of bludgers, Harry!"
"Right," says Harry, who thinks being bouncy wouldn't help with the probable effects of bludger-on-skull impact. But logic, quidditch, and Oliver Wood can never all be in the same room at once, so he lets it go.
Almost dying at a quidditch match doesn't even give him another chance to test his bounciness. When Harry's broom tries its level best to throw him off it can't even manage that right, and he still catches the snitch, so there, stupid broom or stupid curse or stupid Snape or whatever.
It can't be Snape. It really, definitely can't, because Snape isn't anywhere near him the next incident, which involves a Herbology practical, a rack of pots collapsing on top of Professor Sprout, and a Nepalese strangling vine mispotted with the lady's hair ferns they're pruning. Ron yells. Hermione goes at the vine with pruning shears. But it keeps slipping off and can't get a proper grip on Harry. It goes for Hermione next, and gets a solid grip too before she and Harry cut it off at the roots and it dies.
Hermione gets bruises. Harry does not.
"I reckon you're just the unluckiest bloke ever," is Ron's contribution.
This makes sense to Harry. Only the worst luck in the world could have gotten him saddled with the Dursleys.
They have to revisit the Snape theory after the Chrismtas hols: Harry's minding his own business at lunch when, out of nowhere, he falls to the ground yelling as pain bores a hole through his stomach. Then he convulses, burps out a puff of unpleasantly mustardy smoke, and feels fine.
Naturally this sends him to Madam Pomfrey, who ascertains that Harry's food was poisoned, and his magic somehow negated and expelled it from his body before it could do any damage.
As soon as Madam Pomfrey tells them this and goes away, Hermione glares at both boys. "Don't you bloody dare."
Ron and Harry were definitely thinking it. Ron and Harry fall over themselves agreeing: Hermione saying a bad word is scary.
Anyway. Poison = Snape. Duh. Between that and Snape's general evilness, Harry's pretty convinced. Right up until he finds Quirrell hiding Voldemort on the back of his stinky head.
"See! It wasn't just bad luck!" he insists, later.
"Mate," says Ron, "You-Know-Who is out to get you."
That, Harry concedes, might actually be unluckier than having Dudley for a cousin.